I think i understand what's happened as far as the customer is concerned!! The OP isn't registered with a scheme provider, so she thinks he isn't a qualified electrician (typical). Adding insult to injury (in her mind) he has gone through LABC which has added extra cost to the CU change.
Any costs involved with the OP's company re checking his work etc, should be borne by the complaining client, as that is a contract between the customer and the company, and nothing whatsoever to do with the OP!! Any checking of suspected stolen material should be at the companies cost, again nothing to do with the OP unless it's proven that stolen materials were found to be used!!